The invention relates to a process for producing foams based on polyisocyanate polyaddition products by reacting isocyanates with compounds which are reactive toward isocyanates and have a molecular weight of from 400 to 8000 in the presence of blowing agents, catalysts and, if desired, chain extenders and/or crosslinkers having a molecular weight of &lt;400, auxiliaries and/or additives, and also to foams based on polyisocyanate polyaddition products which can be produced by this process.
The production of foams based on polyisocyanate polyaddition products, usually polyurethane and possibly polyisocyanurate products, by reacting isocyanates with compounds which are reactive toward isocyanates in the presence of blowing agents, catalysts and, if desired, auxiliaries and/or additives has been described many times. To produce foams based on polyisocyanate polyaddition products, use is usually made of aromatic diisocyanates which are reacted with suitable compounds which are reactive toward isocyanates. Disadvantages of foams produced in this way are that they have the tendency to discolor under the action of light and during storage and also, in some applications, their hydrophobic character.
Lightfast foams are usually produced on the basis of aromatic isocyanates and UV stabilizers. A disadvantage here is that these UV stabilizers are extremely expensive and are effective for only a limited time. Foams produced from aliphatic isocyanates have hitherto been known only for very specific applications. WO 89/05830 describes polyurethane networks prepared from polyesterols and lysine diisocyanate. However, the preparation of such polyurethanes is complicated and for some applications impossible. Polyurethanes based on aliphatic isocyanates are also described in U.S. Pat. No. 4,018,636, U.S. Pat. No. 4,263,070 and U.S. Pat. No. 3,281,378, but these documents contain no usable teachings on the production of foamed polyurethanes.
Foamed or unfoamed polyurethanes based on aliphatic isocyanates are described in EP-A 210 566 and EP-A 275 010. Disadvantages of these technical teachings are the high content of volatile monomeric aliphatic isocyanates (EP-A 275 010) or the low content of isocyanate groups (EP-A 210 566), which are required for the blowing reaction with water.